Method for producing silicon ingot having directional solidification structure and apparatus for producing the same

ABSTRACT

A method for producing a silicon ingot having a directional solidification structure comprising the steps of: placing a silicon raw material into a crucible of a melting device constructed by mounting a chill plate on an underfloor heater, mounting a crucible with a large cross-sectional area on the chill plate, providing an overhead heater over the crucible, and surrounding the circumference of the crucible with a heat insulator; heat-melting the silicon raw material by flowing an electric current through the underfloor heater and overhead heater; chilling the bottom of the crucible by halting the electric current through the underfloor heater after the silicon raw material has been completely melted to form a molten silicon; chilling the bottom of the crucible by flowing an inert gas through the chill plate; and intermittently or continuously lowering the temperature of the overhead heater by intermittently or continuously decreasing the electric current through the overhead heater, and an apparatus for producing the silicon ingot.

This appln. is a division of Ser. No. 09/257,037 filed Feb. 25, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for producing a silicon ingothaving a directional solidification structure with a suitable degree oforientation for producing a silicon substrate for use in photovoltaicsolar cells, especially to a method for producing a silicon ingot havinga directional solidification structure with a wide horizontalcross-sectional area and good degree of orientation.

2. Description of the Related Art

Silicon substrates comprising polycrystalline silicon have been known inthe art as a kind of silicon substrate for use in photovoltaic solarcells. The silicon substrate comprising the polycrystalline silicon isproduced by slicing a silicon ingot having a directional solidificationstructure. Although the silicon ingot having a directionalsolidification structure is thought to be cheaper than the singlecrystal silicon, many researches are under way for satisfyingrequirements for much cheaper silicon substrates.

FIG. 5 denotes an illustrative cross section for describing the methodfor producing a conventional silicon ingot having a directionalsolidification structure. As shown in FIG. 5(a), the method comprisesthe steps of filling a raw silicon material 2 in a melting crucible 1,melting the raw silicon material 2 by heating the melting crucible 1with an induction coil 3, and injecting the molten silicon 8 into asolidification crucible 4 as shown in FIG. 5(b). An insulating heater 5is provided around the solidification crucible 4 and a baffle 6 forshielding the heat from the heater 5 is additionally provided at thebottom end of the insulating heater 5. A chill plate 7 makes contactwith the bottom of the solidification crucible 4. The injected moltensilicon in the solidification crucible 4 starts to solidify from thebottom to the top since the bottom of the solidification crucible 4 ischilled with the chill plate 7. An elevator shaft 11 is further providedat the bottom face of the chill plate 7, a directional solidificationstructure 12 being grown over the entire region of the molten siliconliquid by allowing the molten silicon 8 to solidify from the bottomwhile the chill plate 7 is descending using the elevator shaft 11 at avelocity synchronizing with the crystal growth speed of the moltensilicon. The silicon substrate for use in the photovoltaic solar cell isproduced by slicing the silicon ingot after shaving off the side wallpart of the silicon ingot, since the side wall of the silicon ingothaving a directional solidification structure obtained as describedabove contains a high concentration of impurities introduced from thecrucible 4 as well as a lot of distortion fault.

However, when the silicon ingot obtained has a small horizontalcross-sectional area, the horizontal cross-sectional area of the siliconingot after shaving off the side wall part becomes still smaller alongwith increasing the silicon ingot side wall elimination ratio, making itimpossible to effectively utilize the expensive silicon raw material.

Accordingly, a silicon ingot having a directional solidificationstructure with a wide horizontal cross-sectional area as well as anexcellent degree of orientation is desired. However, when one attemptsto produce a silicon ingot having a wide horizontal cross-sectional areaby the conventional production method in which the temperature iscontrolled by disposing heat generation sources on side faces, thedegree of orientation along the vertical (i.e., solidification)direction becomes poor due to temperature differences caused along thehorizontal direction, making it impossible to obtain a silicon ingothaving a suitable degree of orientation.

SUMMARY OF THE INVENTION

Accordingly, the object of the present invention is to provide a siliconingot having a good degree of orientation with a wider horizontalcross-sectional area than the conventional silicon ingot.

In one aspect, the present invention provides a method for producing asilicon ingot having a directional solidification structure comprisingthe steps of: placing a silicon raw material into a crucible of amelting device constructed by mounting a chill plate capable of chillingwith a refrigerant on an underfloor heater, mounting the crucible on thechill plate, providing an overhead heater over the crucible, andsurrounding the circumference of the crucible with a heat insulator;heat-melting the silicon raw material by flowing an electric currentthrough the underfloor heater and overhead heater while haltingrefrigerant feed to the chill plate, followed by halting the electriccurrent or decreasing the electric power through the floor heater afterthe silicon raw material has been completely melted, the molten siliconbeing chilled from the bottom of the crucible by chilling the chillplate by feeding the refrigerant; and intermittently or continuouslylowering the temperature of the overhead heater by intermittently orcontinuously decreasing electric current through the overhead heateralong with halting the electric current or decreasing the electric powerthrough the floor heater.

In another aspect, the present invention provides an apparatus forproducing a silicon ingot having a directional solidification structure,where the apparatus is equipped with an underfloor heater, a chill platemounted on the floor heater, a crucible mounted on the chill plate, anoverhead heater provided over the crucible and a heat insulatorsurrounding the circumference of the crucible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 denotes an illustrative cross section showing an apparatus forproducing a silicon ingot having a directional solidification structureaccording to the present invention;

FIG. 2 shows a cross section of the apparatus along the line A—A in FIG.1;

FIG. 3 denotes an illustrative cross section showing an apparatus forproducing a silicon ingot having a directional solidification structureaccording to the present invention;

FIG. 4 denotes an illustrative cross section showing an apparatus forproducing a silicon ingot having a directional solidification structureaccording to the present invention; and

FIG. 5 denotes an illustrative cross section showing an apparatus forproducing a silicon ingot having a directional solidification structureand the method for melting the silicon ingot according to theconventional example.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As shown in FIG. 1, the silicon raw material is placed into a crucibleof a melting device constructed by mounting a chill plate 15 on aunderfloor heater 13, mounting a crucible 14 having a wide horizontalcross-sectional area on the chill plate 15, providing an overhead heater16 over the crucible 14, and surrounding the circumference of thecrucible 14 with a heat insulator 17. The silicon raw material isheat-melted by flowing an electric current through the underfloor heater13 and overhead heater 16. A silicon ingot having a directionalsolidification structure can be produced by halting the electric currentor reducing the electric power after completely melting the silicon rawmaterial to form the molten silicon 8, followed by flowing an inert gasthrough the chill plate 15 to chill the molten silicon from the bottomof the crucible by chilling the bottom of the crucible, along withintermittently or continuously decreasing the temperature of theoverhead heater 16 by intermittently or continuously reducing theelectric current flow through the overhead heater 16.

An underfloor heater 13, a chilled plate 15 mounted on the underfloorheater 13, a crucible 14 mounted on the chilled plate 15, an overheadheater 16 provided over the crucible and a heat insulator 17 around thecircumference of crucible are provided in the apparatus for producingthe silicon ingot having a directional solidification structure.

A heat insulator mainly comprising fibrous carbon is used for the heatinsulator 17 surrounding the circumference of the crucible 14. Whensilicon is melted in a crucible containing silica, SiO is formed by thefollowing reaction:

SiO₂+Si→2SiO

SiO and fibrous carbon undergo a reaction represented by the followingformula:

SiO+2C→SiC+CO

The CO gas generated reacts with molten Si by the reaction representedby the following formula, leaving SiC in the molten Si to sometimes forma silicon ingot in which SiC is left behind.

CO+Si(1)→[O]↑+SiC

A large quantity of SiC remains especially when a thick silicon ingot isproduced because molten state of silicon is required to be kept for along period of time. It is not preferable to produce a photovoltaicsolar cell using a silicon ingot in which SiC is left behind sincephotovoltaic conversion efficiency is deteriorated. Accordingly, inorder to prevent SiC from contaminating the silicon ingot, it isnecessary that the CO gas generated by reacting with carbon in the heatinsulator is prevented from contacting and reacting with the moltensilicon in the crucible by maintaining an inert gas atmosphere in thecrucible during melting of silicon. Consequently it is more preferableto maintain the inert gas atmosphere in the crucible during melting ofsilicon by feeding an inert gas from an inert gas feed device 23 intothe crucible as shown in FIG. 3.

The method for producing the silicon ingot having a directionalsolidification structure as shown in FIG. 3 comprises the steps of:

placing a silicon raw material into a crucible 14 of a melting deviceconstructed by mounting a chill plate 15 capable of chilling with arefrigerant on an underfloor heater 13, mounting the crucible 14 on thechill plate 15, providing an overhead heater 16 over the crucible 14,and surrounding the circumference of the crucible 14 with a heatinsulator 17;

heat-melting the silicon raw material by flowing an electric currentthrough the underfloor heater 13 and overhead heater 16 while haltingrefrigerant feed to the chill plate 15, followed by halting the electriccurrent or decreasing the electric power through the underfloor heater13 after allowing the silicon raw material to completely melt, themolten silicon being chilled from the bottom of the crucible by chillingthe chill plate 15 by feeding the refrigerant; and

intermittently or continuously lowering the temperature of the overheadheater 16 by intermittently or continuously decreasing the electriccurrent through the overhead heater 16 along with halting the electriccurrent or decreasing the electric power through the floor heater.

The apparatus for producing the silicon ingot having a directionalsolidification structure is equipped with an underfloor heater 13, achill plate 15 mounted on the underfloor heater 13, a crucible 14mounted on the chill plate 15, an overhead heater 16 provided over thecrucible, an heat insulator surrounding the circumference of thecrucible and a gas feed device 23 for feeding an inert gas into thecrucible.

The surface of the molten silicon is preferably maintained in a liquidstate in order to sufficiently allowing the molten silicon in thecrucible to uniaxially solidify. Therefore, it is preferable to maintainthe atmosphere in the crucible under a high temperature inert gasatmosphere heated at a temperature higher than the melting point ofsilicon. Accordingly, an inert gas heating device 27, having apre-heating chamber 24 inside of which a pre-heater 25 for heating theinert gas fed from the inert gas feed device 23 is preferably providedas shown in FIG. 3, thereby feeding the inert gas heated with the inertgas heating device 27 into the crucible through a discharge pipe 26.Especially when a temperature sensor 28 is provided at the dischargepipe 26, the temperature of the inert gas flowing out of the dischargepipe 26 can be readily controlled with a program synchronized with thetemperature of the overhead heater 16. Since the discharge pipe isalways exposed to high temperature, it is preferably made of a heatresistant pipe; pipes made of Mo, C, Al₂O₃ and SiC being especiallypreferable as a heat resistant pipe.

As shown in FIG. 3, the silicon ingot having a directionalsolidification structure is also produced by the steps comprising:

placing a silicon raw material into a crucible 14 of a melting deviceconstructed by mounting a chill plate 15 capable of chilling with arefrigerant on an underfloor heater 13, mounting the crucible 14 on thechill plate 15, providing an overhead heater 16 over the crucible 14,and surrounding the circumference of the crucible 14 with a heatinsulator 17;

heat-melting the silicon raw material in the crucible maintained under ahigh temperature inert gas atmosphere heated at a temperature higherthan the melting point of silicon by flowing an electric current throughthe underfloor heated 13 and overhead heater 16 while haltingrefrigerant feed to the chill plate 15, followed by halting the electriccurrent or decreasing the electric power through the underfloor heater13 after completely melting the silicon raw material along with chillingthe chill plate 15 by feeding the refrigerant to chill the moltensilicon from the bottom of the crucible; and

intermittently or continuously decreasing the temperature of theoverhead heater 16 by intermittently or continuously reducing theelectric current through the overhead heater 16 along with halting theelectric current or decreasing the electric power through the underfloorheater.

The apparatus for producing the ingot having a directionalsolidification structure is provided with an underfloor heater 13, achill plate 15 mounted on the underfloor heater 13, a crucible 14mounted on the chill plate 15, a overhead heater 16 provided over thecrucible, an inert gas feed device 23 for feeding an inert gas to thecrucible and an inert gas pre-heating device 27.

While water or an inert gas (Ar and He are preferable as an inert gas)are preferably used for the refrigerant to be fed to the chill plate 15in FIG. 1 and FIG. 3, the inert gas used for chilling the chill plate 15is allowed to be fed into the crucible through a gas passage 29 as shownin FIG. 4 when the inert gas is used for the refrigerant for chillingthe chill plate 15, thereby allowing the inert gas used for chilling thechill plate 15 to recycle for maintaining an inert gas atmosphere in thecrucible. The silicon raw material in the crucible is heat-melted byflowing an electric current through the underfloor heater 13 andoverhead heater 16 while turning the valve 31 to the position asindicated in FIG. 4. A small quantity of the inert gas is fed into thecrucible through the gas passage 29 so that the chill plate is notchilled during heat-melting of the silicon raw material. A hightemperature inert gas atmosphere in the crucible can be maintained byfeeding the inert gas into the crucible after heating the gas with thepre-heater 30.

When the silicon raw material has been completely melted, the electriccurrent through the underfloor heater is halted or the electric power isdecreased along with feeding a large amount of the inert gas to thechill plate to cool the chill plate 15 in order to chill the moltensilicon from the bottom of the crucible. Most of the inert gas used forchilling the chill plate 15 is discharged from an outlet 32 by switchingthe position of the valve 31′ but a part of the inert gas used forchilling the chill plate is fed into the crucible through the gaspassage 29 while heating the gas with a pre-heater 30 integrated aroundthe gas passage 29, thereby allowing the high temperature inert gasatmosphere to be maintained in the crucible. Only a part of the inertgas used for chilling the chill plate is recycled because, when the feedvolume of the inert gas is too large, the gas can not be kept at a hightemperature.

Accordingly, the silicon ingot having a directional solidificationstructure is produced, as shown in FIG. 4, by the steps comprising:

placing the silicon raw material into a crucible of the melting deviceconstructed by mounting a chill plate 15 capable of chilling with theinert gas on the underfloor heater 13, mounting the crucible 14 on thechill plate 15, providing an overhead heater 16 over the crucible 14 andsurrounding the circumference of the crucible 14 with a heat insulator17;

feeding a small amount of the inert gas with heating into the cruciblethrough the gas passage 29 so that the chill plate 15 is not chilledwith the gas along with heat-melting the silicon raw material in thecrucible by flowing an electric current through the underfloor heater 13and overhead heater 16, followed by halting the electric current ordecreasing the electric power through the underfloor heater 13 aftercompletely melting the silicon raw material along with chilling thechill plate 15 by feeding the inert gas to chill the molten silicon fromthe bottom of the crucible, a part of the inert gas used for chillingthe chill plate 13 being fed into the crucible to maintain an inert gasatmosphere in the crucible; and

intermittently or continuously lowering the temperature of the overheadheater by intermittently or continuously reducing the electric currentthrough the overhead heater along with halting the electric current ordecreasing the electric power through the underfloor heater.

The apparatus for producing the silicon ingot having a directionalsolidification structure is provided with an underfloor heater 13, achill plate 15 mounted on the underfloor heater 13, a crucible 14mounted on the chill plate 15, an over head heater 16 provided over thecrucible 14 and a heat insulator 17 surrounding the circumference of thecrucible 14, further provided with a gas passage 29 for flowing theinert gas for recycling the inert gas used for chilling the chill plateand a preheater 30 integrated around the passage 29 for heating therecycling inert gas.

The present invention will be described in more detail referring to FIG.1 to FIG. 4.

The apparatus for producing the silicon ingot shown in FIG. 1,constructed by mounting a chill plate 15 on an underfloor heater 13,mounting a crucible 14 with large cross-sectional area on the chillplate 15, providing an overhead heater 16 over the crucible 14 andsurrounding the circumference of the crucible 14 with a heat insulator17, is placed in a chamber (not shown in the drawing) capable ofcontrolling the atmosphere so as to prevent the silicon raw materialduring melting from being oxidized. The silicon raw material is spreadon the bottom of the crucible 14 and is heat-melted by flowing anelectric current through the underfloor heater 13 and overhead heater16.

The silicon raw material is allowed to completely melt to form a moltensilicon 8, which is then cooled from the bottom of the crucible byhalting the electric current or reducing the electric power through theunderfloor heater 13 along with chilling the bottom of the crucible byflowing the refrigerant through the chill plate 15 to generate adirectional solidification structure 12. Meanwhile, when the temperatureof the overhead heater 16 is intermittently or continuously lowered byintermittently or continuously reducing the electric current through theoverhead heater 16, the directional solidification structure 12 isfurther grown upward, enabling to produce a silicon ingot having adirectional solidification structure with a wide horizontalcross-sectional area.

The chill plate containing a hollow part 18 has a construction in whicha plurality of props are attached in the hollow part 18 of the chillplate 15 aligned in parallel relation with each other along thedirection perpendicular to the thickness of the chill plate 15. FIG. 2shows a cross section of the manufacturing apparatus along the line A—Ain FIG. 1. As shown in FIG. 1 and FIG. 2, a refrigerant inlet 21 isprovided at the center of the chill plate 15 and a feed pipe 22 isconnected to the refrigerant inlet 21. A plurality of refrigerantoutputs 20 are provided at the side wall of the chill plate 15. Thechill plate 15 provided with the refrigerant inlet 21 at its centerpreferably assumes a disk shape, a plurality of props 19 being disposedin concentric relation so that they are not placed to be adjacent alongthe radius direction with each other. The larger number of the props 19allows chilling efficiency with the refrigerant to be improved, alongwith improving transfer efficiency of the heat from the underfloorheater 13.

While water is usually used for the refrigerant to be fed to the chillplate 15, it is preferable to use an inert gas, most preferably Ar, inthe present invention. When the inert gas is fed to the refrigerantinlet 21 provided at the center of the chill plate 15, the gas flowsthrough the spaces among the props 19 and is discharged from a pluralityof refrigerant outlets 20. Any heater can be used for the underfloorheater 13 and overhead heater 16, provided that the heaters are able toheat along a plane surface, and the structure and kind of the heater arenot especially limited. A carbon heating element processed into a flatshape is preferably used for the underfloor heater 13 and overheadheater 16 in the apparatus for producing the silicon ingot according tothe present invention.

Although a heat insulator comprising fibrous carbon is frequently usedfor the heat insulator 17, SiC is possibly formed when silicon is meltedin a silica crucible insulated with the fibrous carbon. Because thesilicon substrate produced from an ingot in which SiC remains has poorphotovoltaic conversion efficiency, it is preferable that an inert gasfeed device for feeding an inert gas in the crucible is provided inorder to maintain an inert gas atmosphere in the crucible while siliconis melting, thereby preventing SiC from forming.

FIG. 3 shows an apparatus for producing the silicon ingot having adirectional solidification structure according to the present invention,wherein an inert gas feed device for feeding an inert gas into thecrucible and an inert gas pre-heating device are further added to theapparatus shown in FIG. 1. Descriptions with respect to the functions ofthe parts other than the inert gas feed device 23 and inert gaspre-heating device 27 in the apparatus shown in FIG. 3 are omittedherein since the functions of the apparatus are the same as those of theapparatus in FIG. 1 except the inert gas feed device 23 and inert gaspre-heating device 27. The inert gas fed from the inert gas feed device23 is fed into the crucible after being heated with the inert gaspre-heating device 27 as shown in FIG. 3, maintaining a high temperature(the temperature to allow silicon to melt, preferably about 1450° C. toabout 1600° C.) inert gas atmosphere in the crucible. A preheater 25 forheating the inert gas fed from the inert gas feed device 23 and apre-heating chamber 24 are provided in the inert gas pre-heating device27, the inert gas pre-heated in the inert gas pre-heating device 27being fed into the crucible through the discharge pipe 26.

FIG. 4 shows an apparatus for producing the ingot having a directionalsolidification structure according to the present invention, whereinmeans for feeding a part of the inert gas used for chilling the chillplate 15 into the crucible after heating as well as for maintaining aninert gas atmosphere in the crucible by recycling the inert gas areadded to the apparatus shown in FIG. 1. The functions of the part of theapparatus in FIG. 4 other than the part used for recycling the inert gasare omitted herein since the functions are the same as those of theapparatus in FIG. 1 except the part used for recycling the inert gas.

In the apparatus for producing the silicon ingot having a directionalsolidification structure shown in FIG. 4, a small amount of the inertgas not allowing the chill plate 15 to be chilled is fed into thecrucible from the feed pipe 22 through the gas passage 29 while siliconin the crucible is heat-melting by flowing an electric current throughthe underfloor heater 13 and overhead heater 16. The inert gas to be fedfor the purpose above is fed into the crucible again after heating witha pre-heating coil 30. After allowing the silicon raw material tocompletely melt, the molten silicon is chilled from the bottom of thecrucible by halting the electric current or reducing the electric powerthrough the underfloor heater along with by feeding a large amount ofthe inert gas to the chill the molten silicon from the bottom of thecrucible by chilling the chill plate 15. Most of the inert gas used forchilling the chill plate 15 is discharged from the outlet 32 but a smallportion of the inert gas used for chilling the chill plate 15 is fedinto the crucible after heating the gas with the preheating coil 30integrated around the gas passage 29. An appropriate valve operationsuch as switching the position of the valve 31′ allows a small portionof the inert gas used for chilling the chill plate to be fed to the gaspassage 29.

Example 1

An apparatus for producing the ingot as shown in FIG. 1 was prepared bymounting a chill plate 15 having a hollow part 18 on an underfloorheater 13, mounting a tray-shaped silica crucible 14 with a depth of 300mm, an inner diameter of 400 mm and an outer diameter of 450 mm on thehollow chill plate 15, providing an overhead heater 16 over the silicacrucible 14 and surrounding the circumference of the crucible 14 with aheat insulator 17.

A large horizontal cross-sectional area silicon ingot having adirectional solidification structure with an inner diameter of 200 mmand outer diameter of 400 mm was produced by the steps comprising:placing a silicon raw material in a tray-shaped silica crucible 14equipped in the apparatus for producing the silicon ingot; heat-meltingthe silicon raw material by flowing an electric current through anunderfloor heater 13 and overhead heater 16 after maintaining an Ar gasatmosphere in the crucible; halting the electric current of theunderfloor heater 13 followed by flowing the Ar gas in order to chillthe molten silicon from the bottom of the crucible by flowing Ar gasthrough the chill plate 15 having a hollow part 18; and continuouslylowering the temperature of the overhead heater 16 by continuouslyreducing the electric current through the overhead heater 16.

The degree of orientation of this large horizontal cross-sectional areasilicon ingot having a uniaxially oriented polycrystallinesolidification texture was evaluated, and a square-shapedpolycrystalline silicon substrate with a dimension of 150 mm×150 mmprocessed from the foregoing ingot was integrated into a solar cell tomeasure its photovoltaic conversion efficiency. The results ofevaluation and measurements are listed in TABLE 1.

Conventional Example 1

The silica crucible with a depth of 300 mm, an inner diameter of 400 mmand an outer diameter of 450 mm was mounted on a chill plate 7 shown inFIG. 5(b). A large horizontal cross-sectional area silicon ingot havinga directional polycrystalline solidification structure with the samethickness of 20 b mm and outer diameter of 400 mm as in Example 1 wasproduced by the steps comprising: melting the silicon raw material in aconventional silica crucible 1 shown in FIG. 5(a); pouring the moltensilicon obtained into the tray-shaped silica crucible as describedabove; descending the tray-shaped silica crucible filled with the moltensilicon at a speed synchronizing with the crystal growth speed of themolten silicon using an elevator shaft 11 together with the chill plate7; and allowing a directional solidification structure 12 to grow overthe entire region of the molten silicon. The degree of orientation ofthis large horizontal cross-sectional area silicon ingot having adirectional polycrystalline solidification structure was evaluated, anda polycrystalline silicon substrate subjected to cutting processing ofthe foregoing ingot was integrated into a solar cell to measure itsphotovoltaic conversion efficiency. The results of evaluation andmeasurements are listed in TABLE 1.

Example 2

As shown in FIG. 3, a wide horizontal cross-sectional area silicon ingothaving a directional solidification structure with an inner diameter of200 mm and outer diameter of 400 mm was produced by the stepscomprising: heating the Ar gas fed from an inert gas feed device 23 at atemperature of 1500° C. with an inert gas pre-heating device 23;completely melting the silicon raw material by flowing an electriccurrent through an underfloor heater 13 and overhead heater 16 whilecontinuously feeding the heated Ar gas into the crucible; halting theelectric current of the underfloor heater 13 followed by flowing the Argas to the chill plate 15 having a hollow part 18 to chill the moltensilicon from the bottom of the crucible; and continuously lowering thetemperature of the overhead heater 16 by continuously reducing theelectric power through the overhead heater 16.

The degree of orientation of this wide horizontal cross-sectional areasilicon ingot having a directional polycrystalline solidificationstructure was evaluated, and a square-shaped polycrystalline siliconsubstrate with a dimension of 150 mm×150 mm processed from the foregoingingot was integrated into a solar cell to measure its photovoltaicconversion efficiency. The results of evaluation and measurements arelisted in TABLE 1.

Example 3

A molten silicon 8 was produced by completely melting the silicon rawmaterial by flowing an electric current through an underfloor heater 13and overhead heater 16 as shown in FIG. 4. The electric current throughthe underfloor heater 13 was halted followed by chilling the bottom ofthe silica crucible by flowing the Ar gas through a chill plate 15having a hollow part 18 to chill the molten silicon from the bottom ofthe crucible. The temperature of an overhead heater 16 was continuouslylowered by continuously reducing the electric current through theoverhead heater 16. An amount of the Ar gas as small as not to chill thechill plate was fed from a feed pipe 22 and the gas was discharged froma refrigerant exit 20 through the hollow part 18 of the chill plate 15.An Ar gas atmosphere was maintained in the crucible by feeding the Argas into the crucible while heating the gas at a temperature of 1500° C.with a pre-heating coil 30 by passing through the gas passage 29. Asmall portion of the Ar gas used for chilling the chill plate 15 was fedinto the crucible through the gas passage 29 while heating the gas atthe same temperature as the temperature of the overhead heater with thepre-heater 30 during the period of chilling the bottom of the silicacrucible, thereby maintaining an Ar atmosphere in the crucible.

A wide cross-sectional area silicon ingot with a thickness of 200 mm andan outer diameter of 400 mm having a directional polycrystallinesolidification structure was produced by the method as described above.The degree of orientation of this silicon ingot was evaluated. Asquare-shaped polycrystalline silicon substrate with a dimension of 150mm×150 mm processed from the foregoing ingot was integrated into a solarcell to measure its photovoltaic conversion efficiency. The results ofevaluation and measurements are listed in TABLE 1.

Photovoltaic Orientation of directional conversion Kind of ingotsolidified structure efficiency (%) Silicon ingot obtained in good 13.2Example 1 Silicon ingot obtained in good 15.2 Example 2 Silicon ingotobtained in good 14.1 Example 2 Silicon ingot obtained in insufficient11.0 Conventional Example

As evident from the results listed in TABLE 1, the wide cross-sectionalarea silicon ingot having a directional solidification structureproduced according to the present invention has a suitable degree oforientation. In addition, a polycrystalline silicon substrate having anexcellent photovoltaic conversion efficiency can be produced from thissilicon ingot, whereas a silicon ingot having a polycrystallinesolidification structure can not be produced by the conventional method,making it impossible to produce a polycrystalline silicon ingot havingan excellent photovoltaic conversion efficiency.

The present invention provides a silicon ingot having a directionalsolidification structure with a low production cost using a crucible,being especially effective in producing a wide horizontalcross-sectional area silicon ingot having a directional solidificationstructure with a low production cost using a crucible having a widecross-sectional area.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

The disclosures of the priority documents, Patent Application Nos.10-043526 and 11-029927, which were filed in Japan on Feb. 25, 1998, andFeb. 8, 1999, respectively, are incorporated by reference herein intheir entireties.

What is claimed is:
 1. A method for producing a silicon ingot having adirectional solidification structure, the method comprising: placing asilicon raw material into a crucible of a melting device constructed bymounting a chill plate on an underfloor heater, mounting the crucible onthe chill plate, providing an overhead heater over the crucible, andsurrounding a circumference of the crucible with a heat insulator;heat-melting the silicon raw material by flowing electric currentthrough the underfloor heater and the overhead heater without feeding arefrigerant to the chill plate, followed by halting or decreasing theelectric current through the underfloor heater after the silicon rawmaterial has been completely melted, the molten silicon being chilledfrom a bottom of the crucible by chilling the chill plate by feeding therefrigerant; and intermittently or continuously lowering a temperatureof the overhead heater by intermittently or continuously decreasing theelectric current through the overhead heater along with halting theelectric current or decreasing the electric power through the underfloorheater.
 2. The method according to claim 1, wherein the chill plate hasa hollow part and a plurality of props attached to the chill plate inthe hollow part with a length of each of the props oriented parallel toa thickness of the chill plate.
 3. A method for producing a siliconingot having a directional solidification structure, the methodcomprising: placing a silicon raw material into a crucible of a meltingdevice constructed by mounting a chill plate on an underfloor heater,mounting the crucible on the chill-plate, providing an overhead heaterover the crucible, and surrounding a circumference of the crucible witha heat insulator; heat-melting the silicon raw material in the cruciblemaintained under an inert gas atmosphere by flowing electric currentthrough the underfloor heater and the overhead heater without feeding arefrigerant to the chill plate, followed by halting or decreasing theelectric current through the underfloor heater after the silicon rawmaterial has been completely melted, the molten silicon being chilledfrom a bottom of the crucible by chilling the chill plate by feeding therefrigerant; and intermittently or continuously lowering a temperatureof the overhead heater by intermittently or continuously decreasing theelectric current through the overhead heater along with halting theelectric current or decreasing the electric power through the underfloorheater.
 4. The method according to claim 3, wherein the inert gasatmosphere in the crucible is heated to a temperature range of about1450° C. to about 1600° C.
 5. The method according to claim 4, whereinthe chill plate has a hollow part and a plurality of props attached tothe chill plate in the hollow part with a length of each of the propsoriented parallel to a thickness of the chill plate.
 6. The methodaccording to claim 3, wherein the chill plate has a hollow part and aplurality of props attached to the chill plate in the hollow part with alength of each of the props oriented parallel to a thickness of thechill plate.
 7. A method for producing a silicon ingot having adirectional solidification structure, the method comprising: placing asilicon raw material into a crucible of a melting device constructed bymounting a chill plate on an underfloor heater, mounting the crucible onthe chill plate, providing an overhead heater over the crucible, andsurrounding a circumference of the crucible with a heat insulator;heat-melting the silicon raw material in the crucible by flowingelectric current through the underfloor heater and the overhead heaterwhile maintaining an inert gas atmosphere in the crucible by feeding asmall amount of the inert gas, insufficient to chill the chill plate,into the crucible through a gas passage, followed by halting ordecreasing the electric current through the underfloor heater after thesilicon raw material has been completely melted along with chilling themolten silicon from a bottom of the crucible by feeding an amount of theinert gas to the chill plate, maintaining the inert gas atmosphere inthe crucible by feeding a part of the inert gas used for chilling thechilling plate into the crucible; and intermittently or continuouslylowering a temperature of the overhead heater by intermittently orcontinuously decreasing the electric current through the overhead heateralong with halting the electric current or decreasing the electric powerthrough the underfloor heater.
 8. The method according to claim 7,wherein a temperature of the inert gas atmosphere in the crucible ismaintained by feeding a part of the inert gas used for chilling thechill plate into the crucible.
 9. The method according to claim 8,wherein the chill plate has a hollow part and a plurality of propsattached to the chill plate in the hollow part with a length of each ofthe props oriented parallel to a thickness of the chill plate.
 10. Themethod according to claim 7, wherein the chill plate has a hollow partand a plurality of props attached to the chill plate in the hollow partwith a length of each of the props oriented parallel to a thickness ofthe chill plate.